The present invention relates to a magnetic recording and reproducing apparatus having a pair of heads that are used alternately, more particularly to the adjustment of the timing of the switching from one head to another.
An example of this type of magnetic recording and reproducing apparatus is a video cassette recorder having two video heads mounted in a rotating drum so as to scan alternate helical tracks on a magnetic tape. One field of a video signal is recorded in each helical track. The switching between the two heads is controlled by a head-switching signal generated by a bi-stable circuit or flip-flop that changes state once per field. The head-switching must be timed to avoid the vertical synchronizing pulse interval, lest synchronization be disturbed by switching noise, leading to picture problems such as imperfect interlacing and jitter. For similar reasons, the head-switching point should not occur in the visible part of the picture. In video cassette recorders of the widely used VHS type, the head-switching point is positioned six and one-half horizontal scanning periods (6.5H) before the vertical synchronizing pulse.
This positioning is illustrated in FIG. 1, which shows three adjacent helical tracks on a magnetic tape 10. Sv denotes the vertical synchronizing pulse interval. The switching points at the beginning and end of one track are indicated by black dots. FIG. 2 shows an example of the corresponding relationships among the vertical synchronizing pulse Sv, the head-switching signal Ss, and a sensor pulse Sp from which the head-switching signal Ss is generated.
When a video cassette recorder is manufactured, the head-switching point must be adjusted so that it is correctly positioned on the magnetic tape. One conventional means of adjustment is an analog means, such as a monostable multivibrator with a variable resistor, that creates a variable delay from the rise of Sp to the rise of Ss in FIG. 2. FIG. 3 shows a conventional video cassette recorder in which this means is employed.
The video cassette recorder has an external input terminal 1 for a video signal received from other apparatus, an antenna input terminal 2 for input of a signal received from an antenna, an input switching device 3 selecting one of the two input terminals 1, 2, a tuning circuit 4 that converts the antenna signal to a video signal, a signal-processing circuit 5 that separates horizontal and vertical synchronizing signals from the video signal, an amplifier circuit (AMP) 6, and a control unit 7. The control unit 7 controls the rotation of a drum 8 by controlling a drum motor 8a. A pair of video heads 9a, 9b, mounted at diametrically opposite positions on the rotating drum 8, scan a magnetic tape 10, which wraps substantially halfway around the drum 8.
The video cassette recorder has a record button 11 and a play button 12. When the play button 12 is pressed and a picture is reproduced from the magnetic tape 10, the amplifier circuit 6 amplifies the signal picked up by the heads 9a, 9b and supplies the amplified signal to the signal-processing circuit 5. The signal-processing circuit 5 separates the vertical synchronizing signal 14 from the amplified signal, and supplies the vertical synchronizing signal 14 to the control unit 7.
The above components are enclosed within a housing 15, which has a video output terminal 17. Also disposed within the housing 15 is a variable resistor 18 for adjusting the head-switching timing. When a video signal is reproduced from the magnetic tape 10, the reproduced video signal 19 is supplied to the video output terminal 17.
The sensor pulse signal Sp is generated by a permanent magnet 20 mounted on the rotating drum 8 and a position-sensing head 21 that senses the magnetic field of the permanent magnet 20. The control unit 7 comprises a microelectronic computing device such as a microcontroller unit (MCU) 22, which receives the vertical synchronizing signal 14 and signals from the record button 11 and play button 12, and a servo circuit 23, which receives the sensor pulse signal Sp and generates the head-switching signal Ss. The servo circuit 23 uses the head-switching signal Ss in controlling the drum motor 8a. The microcontroller unit 22 uses the head-switching signal Ss to control the head-switching timing by controlling a switch (not visible) in the amplifier circuit 6 or signal-processing circuit 5. The switch is controlled so that, for example, video head 9a is selected when Ss is high, and video head 9b when Ss is low.
The servo circuit 23 includes a flip-flop (not visible) from which the head-switching signal Ss is output, and the above-mentioned monostable multivibrator (not visible), which sets the flip-flop after being triggered by the sensor pulse Sp. The delay from the triggering pulse Sp to the setting of the flip-flop is determined by the variable resistor 18. By setting the flip-flop, the Sp pulse generates the rising transition of the head-switching signal Ss. The servo circuit 23 resets the flip-flop so that the falling transitions of the head-switching signal Ss occur midway between the rising transitions.
The timing of the rising and falling transitions of the head-switching signal Ss requires adjustment to compensate for variations in the mounting positions of the permanent magnet 20 and position-sensing head 21, and variations in the shape of the magnetic field generated by the permanent magnet 20. When the head-switching timing is adjusted, a standard reference cassette is loaded into the video cassette recorder. The magnetic tape 10 in the reference cassette has a pre-recorded reference video signal with vertical synchronizing pulses located in the positions specified by the VHS standard. The person performing the adjustment connects an oscilloscope to test points enabling the head-switching signal Ss and vertical synchronization signal Sv to be monitored simultaneously, presses the play button 12, and adjusts the variable resistor 18 so that the rising and falling transitions of the head-switching signal Ss occur six and one-half horizontal scanning periods before the vertical synchronizing pulse Sv.
When the adjustment process described above is carried out on video-cassette-recorder production lines, it is time-consuming and inconvenient, because it requires the connection of an oscilloscope and manual adjustment of the variable resistor 18. This adds to the cost of the video cassette recorder. Like analog adjustments in general, the adjustment is also inherently imprecise, being subject to human variability, temperature effects, and aging drift.
In a video cassette recorder designed to record and reproduce video signals conforming to different television broadcast standards, a further problem is that even if the switching point is adjusted accurately according to one standard, it may be inaccurate for another standard. For example, 6.5 horizontal scanning periods is equal to substantially four hundred thirteen microseconds in the NTSC standard, but to four hundred sixteen microseconds in the PAL standard. Incidentally, NTSC stands for National Television System Committee, and PAL for phase alternation by line.
As a partial solution to these problems, U.S. Pat. No. 4,914,531 describes a video recorder that, when reproducing a recorded video signal, automatically adjusts the head-switching point in relation to the vertical synchronization signal. This feature does not assure that the head-switching point will be accurately positioned when a video signal is recorded, however, so manual adjustment is also necessary.
Japanese Unexamined Patent Publication Nos. 60-150208 and 5-41850 describe magnetic recording and reproducing devices that adjust the head-switching point automatically and store the adjusted value in a memory. These features eliminate the need for manual adjustment, but fail to provide a suitable initial value, do not provide means of confirming the adjustment, and do not solve the problem of different adjustments being needed for different television standards.
It is accordingly an object of the present invention to simplify and shorten the head-switching adjustment of a magnetic recording and reproducing apparatus.
Another object of the invention is to provide means of confirming the adjustment.
Another object is to provide an accurately adjusted head-switching point when different types of signals are recorded and reproduced.
The invented method of adjusting the head-switching point of a magnetic recording and reproducing apparatus comprises the steps of:
storing an initial value in a memory device;
reproducing a reference video signal, using the initial value to control the head-switching point;
measuring the interval from the head-switching point to the reproduced vertical synchronizing pulse;
calculating a control value that sets the head-switching point a predetermined distance from the vertical synchronizing pulse; and
storing the calculated control value in the memory device for subsequent use in recording video signals.
These steps may be carried out for an NTSC reference video signal, a PAL reference video signal, or both types of reference video signals. The invented method may also comprise one or more of the following steps:
automatically discriminating between NTSC and PAL reference videos signals;
automatically converting the control value between NTSC and PAL modes; and
automatically converting the initial value between NTSC and PAL modes.
The invention also provides a magnetic recording and reproducing apparatus that employs the invented method of adjusting the head-switching point.